Grounding planes after the 11 September attacks may not have caused unusual temperature effects.

When all commercial air traffic in the United States was grounded after the terrorist attacks of 11 September 2001, scientists got an unexpected opportunity to test ideas about the climate effects of the condensation trails left behind by jets.

A study in 2002 suggested that these contrails could have a significant effect on daily temperature patterns (see 'Air-traffic moratorium opened window on contrails and climate'). But a new analysis now claims that altered US temperature patterns during the three flight-free days can be explained by natural variations in cloud cover, rather than the absence of planes.

Aircraft contrails can spread into cirrus-like clouds high in the atmosphere. Similar to natural clouds, they are thought to have an overall warming effect on the planet. But they can also moderate daily temperature extremes by trapping heat that escapes from the ground and reflecting sunlight. This raises the lowest overnight temperatures and, to a lesser degree, reduces the highs during daylight hours, scientists have suggested.

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With air traffic projected to grow by 2—5% per year in the near future — amounting to at least a tripling in traffic by 2050 — the effects of contrails are expected to become an increasingly important factor in climate change. But atmospheric scientists are still unsure about the scale of the contrails' impact.

Theory or fact?

Two studies noted that when planes stopped flying on 11—14 September 2001, the average daily temperature range in the United States rose markedly, exceeding the three-day periods before and after by an average of 1.8 °C. The unusual size of the shift, says David Travis of the University of Wisconsin—Whitewater, who led both of the earlier studies, implied that an absence of contrails gave the temperature range a significant boost. But that idea, he says, was "more like a hypothesis" than a firm conclusion.